Basic Trope: Forcibly changing a subject under discussion (or a group's goals or plans) away from one topic or objective either against the will of the participants or by persuasion, toward one desired by the person changing it.
In a discussion among feminist writers who see the "Not If They Enjoyed It" Rationalization trope as offensive about how to deconstruct it, Bob joins the discussion and begins talking about freedom of speech and how men have the right to write whatever they wish.
A Troll posts to a discussion about Police Brutality saying the victims deserved it (with optional racial slurs) and going on at length over how difficult police officers' work is.
Bob, unable to win any points with his arguments, turns into the Garbage Post Kid and Spammer, finally causing that discussion and any other discussion like it to be shut down simply because of his Nausea Fuel and Triggering images until he gets banned.
The derailed discussion becomes instant Internet Backdraft, with all of the parties involved now screaming at each other and starting other fights outside of it rather than recognizing what had happened.
Downplayed: Someone tries to derail a discussion but their post is ignored or brushed off, with no one really caring about their post or what they had to say.
The original discussion has reached its logical end and the derailing is more due to the Left Fielder showing up and getting bored.
The topic that was derailed onto was actually an important topic in and of itself that developed out of the original discussion.
A reasonable person comes to a venue infested with Trolls, SockPuppets, MeatPuppets, and all types of The Shill to discuss a controversial issue, staying on topic and refusing to get distracted into a Flame War. The residents are so shocked with the on topic and intelligent commentary that it stays relatively peaceful.
A liberal commentator changes a discussion about freedom of speech into one about violence against women, or a discussion about street crime into one about police brutality.
Bob's derailing attempt and his postings to the discussion only served to show just how creepy some fans of rape fantasy fiction who go out of their way to support it can be.
Parodied: The "This thread is now about trains" example from the main page.
The derail itself is derailed.
Something in the derailing attempt connected to the topic, so it only half succeeded - only derailing to a different facet of the original topic. Then somehow that gets derailed to something entirely unrelated, and THAT gets derailed to something somewhat approximating the first point of the original topic.
Comment moderation keeps the discussion on topic.
The majority of the posters downvote obvious derails with self-moderation.
People refuse to take the bait.
"No topic" discussions, spam forums, a certain Image Board, and other places where off topic posting is encouraged.
Invoked: "I think Bob is derailing this discussion".
Exploited: All uses of derailing played straight are also exploited.
When Bob arrives in the discussion with his first post, it is called out as derailing and he is banned.
The Police Brutality discussion had, in the first posting, a rule that anyone outright defending police brutality or using racial slurs would have their posts deleted and they would be banned. When the Troll posted, his post was reported and this happened. The discussion went on relatively peacefully afterward and nothing of value was lost.
Discussed: "If we ever have a thread on Controversial Topic X, let's specify what behavior we expect of the posters and post a link to Derailing for Dummies".
Conversed: "It looks like we have a Troll in here trying to derail this thread. What should we do?"
Someone investigates the racist pro-brutality Troll and finds out that yes, he is actually a police officer instead of being the armchair warrior people suspected. After an Internet Counterattack showing exactly what he thinks of the people he's allegedly "protecting and serving," he is told to Turn in Your Badge and put under investigation by internal affairs.
Despite Bob's issues, he did actually have some very good points about freedom of speech and freedom of the press - he just expressed them very badly and in the wrong venue. In a later discussion about legally banning the new manga "Bastard Boyfriend" over its scene with the titular character unironically using the "Not If They Enjoyed It" Rationalization, he makes a very cogent argument over why the manga should not be banned by law.
The Troll posting to the Police Brutality discussion just happened to be reflecting the thoughts of his entire police department. The Internet Counterattack this time takes it to the national media, and some people end up actually thanking the Troll because without his abusive, angry, racist postings, no one would have known about the massive case of Bad Cop/Incompetent Cop for which he became the unwitting spokesman.
Played For Laughs: Bob's girlfriend is angry at him. Cue an "Oh look Babies!" attempt by Bob to change the subject. It works.
Played For Drama: Bob is making an impassioned plea in front of the corrupt court to save his village from famine. But the corrupt officials just laugh at him and start a petty argument on the low quality of today's cupcakes in front of a powerless and disillusioned Bob.